add support for setting JSX for title on Dialog props

We want to avoid contributors using Dialog and nesting DialogHeader within if they only want the default behaviour. See https://github.com/desktop/desktop/pull/6027#discussion_r229337387 for more discussion.

[niik]

Hey @shiftkey, sorry, I’ve completed missed this.

// TODO: how can we get a string-like representation of a JSX element?
 const id = typeof this.props.title === 'string' ? this.props.title : '???'```

You can ignore this entirely. CreateUniqueId will ensure it’s a unique id regardless of what you pass it so just leave that as-is (ie. pass the raw JSX.Element to it, that’s fine). You can use createUniqueId without giving it anything if you like and it’ll take care of it, we only give it something that’s slightly human readable for sanity’s sake.

[shiftkey]

It looks like that doesn’t actually render the element so the code might not work as described.

To be clear, I’m only looking for a string-like representation of the element for situations where we need something distinct to work with (the original issue was using the title to generate a unique id). I’m not looking to actually render the elements.